Intensity spatial filter having uniformly spaced filter elements

ABSTRACT

A spatial filtering system for inspecting integrated circuit photomasks, and the like. The system includes a spatial filter comprising a matrix-like array of opaque regions on a transparent field. The region-to-region spacing on the filter is uniform and greater than that which results if it is assumed that the lens is perfect and generates an exact Fourier transform at its backfocal plane. As a result, high-frequency spatial information, corresponding to the edges of the photomask features, is suppressed improving the signal-to-noise ratio of the system.

ilnited States Patent 11 1 Heinz et al.

1451 July 3, 1973 INTENSITY SPATIAL FILTER HAVING UNIFORMLY SPACEDFILTER ELEMENTS [75] Inventors: Robert Alfred Heinz, FlemingtonTownship, Hunterdon County; Laurence Shrapnel] Watkins, HopewellTownship, Mercer County, both of NJ.

[73] Assignee: Westinghouse Electric Company,

Incorporated, New York, NY.

22 Filed: May 3,1972

21 Appl. No.: 249,985

52 us. C1. 356/71, 350/162 SF, 356/239 51 int. Cl. 00111 21 32, 0021527/38 [58] Field 6: Search also/ 235,35,

356/71, 168, 200,237, 239; 256/219 CR, 219 DF [56] References CitedUNITED STATES PATENTS 3,414,875 12/1968 Driver et a1. 350/162 SF3,630,596 12/1971 Watkins 350/162 SF 3,658,420 4/1972 Axelrod 356/7110/1971 Mathisen 356/71 OTHER PUBLICATIONS Watkins, Proc. of the IEEE,Vol. 57, No. 9, September 1969, Pages 1634-1639. Lohmann et al., AppliedOptics, Vol. 7, No. 4, April Primary Examiner-David Schonberg AssistantExaminer-Ronald J. Stern Attorney-W. M. Kain, B. W. Sheffield 57ABSTRACT A spatial filtering system for inspecting integrated circuitphotomasks, and the like. The system includes a spatial filtercomprising a matrix-like array of opaque regions on a transparent field.The region-to-region spacing on the filter is uniform and greater thanthat which results if it is assumed that the lens is perfect andgenerates an exact Fourier transform at its back-focal plane. As aresult, high-frequency spatial information, corresponding to the edgesof the photomask features, is suppressed improving the signal-to-noiseratio of the system.

6 Claims, 11 Drawing Figures mzmwm 3 ma 3.741423 sum 2 or 4 PAIENTED JUL3 I975 SKHQNQ ORDER INTENSITY SPATIAL FILTER HAVING UNIFORMLY SPACEDFILTER ELEMENTS BACKGROUND OF THE INVENTION 1. Field of the InventionBroadly speaking, this invention relates to spatial filtering. Moreparticularly, in a preferred embodiment, this invention relates to animproved spatial filtering system which inhibits transmission ofsubstantially all periodic information in the filtered image, therebysignificantly improving the signal-to-noise ratio of the systern.

2. Discussion of the Prior Art As is well known, in the manufacture ofintegrated circuits, and the like, wafers of silicon, or othersemiconductor material, are coated with a layer of photoresist and,then, exposed to light through a special photographic plate, known inthe industry as a photomask. The exposed photoresist is then developed,in the conventional manner, and unexposed areas of the photoresist areremoved, thereby, exposing underlying portions of the silicon wafer.These exposed portions are then subjected to processing steps, such asdiffusion, etching, and the like.

A typical IC photomask may comprise a matrix-like array of thousands ofnominally identical photomask features, each in itself a complex patternof lines and other geometric shapes. Such photomasks have heretoforebeen made by successive photographic reductions from a large, hand-mademaster pattern, in a step-andrepeat camera, or more recently, by directexposure of a photographic plate or chromium-coated plate in acomputer-controlled electron-beam machine. More recently still, aprimary pattern generator (PPG), a computer-controlled,electro-mechanical, laser deflection system, has been successfullyemployed to manufacture lC photomasks [See Bell System TechnicalJournal, (Nov. 1970), Vol. 49, No. 9, p. 2,03l-2,076].

However, regardless of the manufacturing process employed, lC photomasksare expensive and time consuming to make. Accordingly, every effort ismade to prolong their useful life. Because of the extremely highresolution required with modern IC devices, exposure ofphotoresist-covered silicon wafers can only be satisfactorilyaccomplished by a contact-printing process, in which the emulsion sideof the photomask is placed in direct physical contact with the wafer.This frequently results in damage to the mask during exposure.Furthermore, pinhole defects may occur during manufacture of thephotomask itself, and dust or dirt may settle on the mask during use.

These defects are, of course, very serious, for any wafer exposed tolight through a damaged or dirty photomask may yield dozens ofdefective, or wholly inoperative, IC devices. This situation is furtheraggravated by the fact that the same photomask is used over and overagain. Thus, a given defect on a mask might be responsible for thousandsof defective IC devices, a most undesirable situation.

As previously discussed, lC photomasks are too expensive to be discardedafter they have been used only a few times. Accordingly, it becomesnecessary to carefully inspect each mask after manufacture and also,somewhat less critically, during actual .production. Heretofore, thisinspection was done manually by a skilled human operator, with the aidof a microscope. However, because of the complex nature of the geometricpattern in each photomask feature, as well as the fact that each maskcontains many thousands of identical features, human error and fatiguehave been found to result in the failure to detect significant numbersof defects.

To overcome this problem, a spatial-filtering technique was developed toinspect the photomasks. This technique forms the subject matter ofcopending U.S. Pat. application, Ser. No. 858,002, filed Sept. 15, 1969,(Watkins Case 1) which application is assigned to the assignee of theinstant invention.

As disclosed in said copending application, the photomask to beinspected is illuminated by spatially coherent radiation from a laserand positioned proximate the front-focal plane of a convex lens. Inaccordance with well-known optical principles, an image will be formedat the rear-focal plane of the lens which corresponds to a Fouriertransform of the photomask. That is to say, the image is a compositediffraction pattern whose spatial distribution is the optical product oftwo components: (1) the interference function of the photomask,comprising a distribution of bright dots of light whose spacing isinversely proportional to the spacing between adjacent features in themask; and (2) the diffraction pattern of a single feature. Now, as,disclosed in said copending application, if a spatial filter comprisingan array of opaque regions on a transparent field, is positionedproximate the back-focal plane of the lens, and if the spacing betweenthe opaque regions corresponds exactly to the spacing between the dotsof light in the diffraction pattern, substantially all of the lightenergy from the laser will be blocked.

However, if the mask is defective in some way, for example, if the maskis scratched, etc., the Fourier transform of the defect will notspatially correspond to the pattern of opaque regions on the filter, andaccordingly, some light will succeed in passing through the filter,thereby enabling the scratch or other defect to be easily detected.

The above-described spatial filtering technique has been highlysuccessful in practice. However, certain problems were encountered whenan attempt was made to automate the inspection process. For example, inorder to eliminate the human factor, a television camera, coupled to acounting device, was positioned'to view the filtered image of the mask.As the camera scanned over the image, the counting device recorded thehumber of defects detected, and, if the value so found exceeded somepredetermined value, the mask was discarded, or set aside for possiblerepair.

The system disclosed in copending application, Ser. No. 858,002,(Watkins Case I), assumed, for the sake of simplicity, that theinterference function produced by a lens comprises equally spaced dots.In practice this is not exactly so, and the lens generates aninterference function in which the dots become progressively furtherapart by very small increments. Furthermore, the lens may suffer fromone or more optical aberrations, such as coma, astigmatism, fieldcurvature, and distortion. The net effect is that, as the light energyimpinges on those parts of the filter which lie further and further awayfrom the center of the filter, the opaque regions thereon no longerfully block the light which is coming from the photomask, even in thetotal absence of defects on the mask. This is so for two reasons: first,the outermost regions are improperly positioned to fully intercept thelight from the photomask, even if it were properly focused on theregions. Secondly, because the opaque regions are physically located ona planar sur face, the outermost opaque regions lie increasingly a smalldistance apart from the true focus of the lens, and hence, in effect,become progressively too small to fully block the light from thephotomask. The outermost regions, of course, are intended to interceptthe higher spatial frequencies from the photomask and, in practice, theonly features on the mask possessing such higher spatial frequencies arethe edges of the photomask features.

In prior art systems, where the filtered image was inspected by a humanoperator, this failure to fully suppress periodic, high frequency, edgeinformation did not prove to be a significant problem. In fact, it wassomewhat of an advantage, because the outline of the individualphotomask features could be seen very faintly in the background of theimage, as viewed by the operator. Thus, the approximate location of thenonperiodic defects which were successfully isolated by the system couldbe rapidly ascertained. However, in an automated process, this no longerholds true, because a television camera does not have a human operatorsability to reason and is unable to discriminate between a true defectand the high-frequency edge information of the photomask features. Thus,in the automated process, the edge information was erroneously countedas a defect, which it is not. An additional problem with the prior artapproach of copending application, Ser. No. 858,002, (Watkins Case 1 isthat, because of the presence of high-frequency edge information, only afew of the thousands of features on a mask can be inspected at the sametime. Now, if an attempt is made to increase the field of view, that isto say, if instead of inspecting only twenty or so of the thousands offeatures on a given mask it is desired to simultaneously examine severalhundred features, the spatial filter must, accordingly, be made withconsiderably more accuracy.

As one solution to this problem, copending application, R. A. Heinz, etal., Ser. No. 249,983, of even date, discloses a spatial filteringtechnique employing a spatial filter wherein the spacing betweenadjacent opaque regions on the filter increases from region-to-region,from the centermost region outward, according to a precise mathematicalformula. This spatial-filtering technique has proved highly successfulin practice. However, the precision with which the opaque regions mustbe positioned on the surface of the filter dictates that such a filterbe made by the use of a computercontrolled device, such as the primarypattern generator or by an electron-beam machine. This makes the filterexpensive and time consuming to produce.

However, not all inspection tasks are so demanding that a mask of thishigh quality is required. Accordingly, it is an object of this inventionto provide a method of spatially filtering an image which suppressessubstantially all periodic information in the image, therebysignificantly enhancing the signal-to-noise ratio of the image, yetwhich utilizes a relatively inexpensive spatial filter having a uniformelement-to-element spacmg.

It is a further object of this invention to provide a spatial filter ofnovel construction for practicing the above method.

SUMMARY OF THE INVENTION To attain these, and other objects, a firstembodiment of the invention comprises a method of isolating nonperiodicerrors in a two-dimensional pattern containing a regular array ofnominally identical elements, mutually spaced apart by a predetermineddistance along at least one axis. The method comprises the steps offirst directing a spatially coherent beam of light at the pattern todiffract the light, and then focusing the diffracted light onto a filtercontaining a plurality of discrete opaque regions on a transparentfield, to spatially modulate the light. The spacing between adjacentregions, along at least one axis of the filter, is uniform and greaterthan the spacing dictated by the equation:

x nltf/d where,

x the distance from the centermost region (the origin) to the n" region;

A the wavelength of said beam of light;

n the order of the spatial harmonic;

d the step-and-repeat of said regular array of elements;

f the focal length of said focusing lens.

Finally, the spatially modulated light is reimaged to form an imageexhibiting the non-periodic errors in the pattern, the filter blockingessentially all periodic information in the image, including the higherspatial frequency components.

For practicing the above method, another embodiment of the inventioncomprises a spatial filter for filtering the Fourier transform of theimage of a workpiece comprising a matrix-like array of nominallyidentical features. The filter comprises a matrix-like array of opaqueregions on a transparent field, the spacing between adjacent regions,along at least one axis of said array, being uniform and greater thanthe spacing dictated by the equation:

x nhf/d where,

x the distance from the centermost region (the origin) to the n" region;

A the wavelength of the light forming said image;

n the order of the spatial harmonic;

d the step-and-repeat of the workpiece;

f the focal length of the image-forming lens.

By the use of this filter, the above-defined opaque regions inhibitfurther transmission of substantially all periodic information in theFourier transform.

The invention and its mode of operation will be more fully understoodfrom the following detailed description, and the following drawing, inwhich:

DESCRIPTION OF THE DRAWING FIG. I is a partially schematic, isometricview of a I first embodiment of the invention;

FIG. 2 illustrates a typical workpiece of the type which may beinspected by the instant invention;

FIG. 3 shows an enlarged view of a portion of the workpiece shown inFIG. 2;

FIG. 4 depicts the format of the diffraction pattern produced when theworkpiece of FIG. 2 is inspected by the apparatus of FIG. 1;

FIG. 5 depicts an illustrative prior art spatial filter;

FIG. 6 is a diagram illustrating the theory underlying the instantinvention;

FIG. 7 is a graph showing the spacingof filtering elements on the filterof FIG. 5, as a function of the spatial harmonic;

FIG. 8 depicts the relative orientation of the filtering elements of aprior art filter and the filter according to this invention;

FIG. 9 is a graph showing the spacing of the opaque regions of thefilter according to this invention;

FIG. 10 is a diagram illustrating the underlying theory of operation ofthe filter according to this invention; and

FIG. 11 is a diagram illustrating the operation of the above filter ingreater detail.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 depicts an illustrativeembodiment of the invention. As shown, the apparatus comprises a laser10 which, when connected to a suitable source of energy (not shown),emits a beam of spatially coherent, radiant energy along a longitudinalaxis 11. The light from laser 10 is directed through a beam expander 12comprising a first lens 13 and pinhole 14. The expanded beam is thenpassed through a collimating lens 16 and finally falls upon the ICphotomask 17 to be inspected.

FIGS. 2 and 3 illustrate photomask 17 in greater detail. As shown, thephotomask comprises a glass photographic plate 18 having recordedthereon a matrix-like array of nominally identical features 19. As shownin FIG. 3, each feature comprises a complex pattern of opaque areas 21on a transparent field, the pattern in each feature defining the areasof the photoresistcovered semiconductor wafer which are to beprotectedfrom exposure to the light. It will be noted that all of the edges ofthe areas 21 in feature 19 are parallel to either the horizontal or tothe vertical axes of the mask. By analogy to the orientation of theblocks in a typical city, such a configuration is frequently referred toas Manhattan geometry, although, of course, the invention is not limitedto inspecting workpieces having such Manhattan geometry and can inspect,with equal success, other workpiece configurations. It will also benoted that, in FIG. 2, a uniform spacing D is assumed to exist betweenthe center lines of each feature on the mask. It is further assumed thatthis spacing is the same in both the horizontal and vertical directions,(i.e., D D). Occasionally, a photomask is produced in which thefeature-to-feature spacing differs in the horizontal and verticaldirections. However, this is easily compensated for in the design of thespatial filter, and the underlying theory of the instant inventionapplies to both arrangements.

In the drawing, mask 17 is depicted as having a 5 X 5 matrix of featuresthereon. One skilled in the art will appreciate that this is merely forconvenience in illustrating the invention and that an actual photomaskmay have as many as 40,000 features thereon arranged in a 200 X 200matrix.

Returning now to FIG. 1', photomask 17 is positioned at the front-focalplane of a second lens 22 which, as previously discussed, will form aFourier transform of the photomask at the back-focal plane thereof. Inaccordance with the teachings of copending application, Ser.No. 858,002,(WatkinsCase l), a spatial filter 23 is positioned at the back-focalplane to intercept all periodic information from photomask l7 and topermit all is connected by a lead 26 to a control circuit 27, whichincludes conventional power supplies, amplifiers, deflection apparatus,etc. A digital-readout device 28 is connected to control circuit 217 bya lead 29 to record the number of defects in photomask 17 which succeedin passing through spatial filter 23 and are detected by camera 25.

FIG. 4 illustrates the pattern which would be seen if a screen were tobe positioned at the back-focal plane of lens 22, rather than spatialfilter 23. For convenience in drawing, this pattern is shown as a seriesof black dots on a white field. It will be appreciated that, in actualpractice, each of the black dots in FIG. 4 represents a spot of brightlight. As seen, the pattern approximates a cross with the spacingbetween adjacent light dots, in the horizontal direction, beinginversely proportional to the feature-to-feature spacing in thehorizontal direction in mask 17. Similarly, the spacing between adjacentdots, in the vertical direction is inversely proportional to thefeature-to-feature spacing in photomask 17 in the vertical direction.If, as discussed, the feature-to-feature spacing on the mask is uniform,and equal, in both directions, then under the assumptions made in thereferenced copending application (Watkins Case I), the dot-to-dotspacing in the diffraction pattern will also be uniform, and equal, inboth directions. The large central dot 31 corresponds to the do term ofthe Fourier transform and, moving to the right, in the horizontaldirection, dot 32 corresponds to the first harmonic," or fundamentalspatial frequency, (i.e., the step-and-repeat pattern of the mask), dot33 the second harmonic, and so on.

FIG. 5 depicts a spatial filter of the type disclosed in theabove-referenced copending application, (Watkins Case I). This filtercomprises an array of opaque regions on a transparent field. This typeof filter can be manufactured by the use of any of several knowntechniques in essentially the same manner that the photomask itself maybe manufactured. Considerable success has been obtained by the use ofthe above-referenced primary pattern generator, and by the use of astepand-repeat camera. If, as is usually the case, thefeature-to-feature spacing on the mask is uniform, and equal, along boththe horizontal and vertical axes, then the array of opaque regions inthe spatial filter will also be uniform, and equal, along both axes, andwill coincide with the location of light spots 31 through 34, etc., inFIG. 4.

While the intensity of light and the size of the spots in the actualdiffraction'pattern of FIG. 4 may vary, the

, opaque regions in FIG. 5 are all uniform in size and non-periodicinformation, such as defects in the photodensity. Of course, the regionsmust be large enough to block the largest of the light spots shown inFIG. 4.

As previously discussed, the system described in copending application,Ser. No. 858,002 (Watkins Case I) assumed that the lens was perfect andproduced equally spaced dots, and this assumption was reasonable for theinspection scheme contemplated by that invention. However, for morecritical applications, this assumption is not valid, and the deviationmust be taken into consideration. In FIG. 6, a lens 41 is shownpositioned so that a diffraction grating 42 is at its frontfocal plane.The diffraction grating has elements spaced apart by a uniform distanced. Typical light rays 43 are shown coming from the diffraction gratingat an angle 0 to the horizontal axis, as shown, and are imaged by lens41 onto the back-focal plane of lens 41.

From basic diffraction theory, it is known that when a plane, collimatedbeam of light is incident upon an intensity grating, the resultingpattern behind the grating is the superposition of many plane waves,each propagating in a different direction. The angle at which thesebeams emanate from the grating is a function of the harmonic which theyrepresent, that is:

sin 0 nit/d where,

)t the wavelength of light;

n the order of the harmonic;

d the step-and-repeat of the array;

f the focal length of the Fourier transform lens. Each of these waves isthen focused to a spot in the back-focal plane by the Fouriertransforming lens 41. The hemispherical surface 44 has been included toaid in computing the location of these images in the plane 45. Thelocation of the light spots on the plane 45 can then be computed fromsimple geometry:

x =ftan 0 =ftan [sin ("M/d] Since for small angles, i.e., low spatialfrequencies, sin 0 5 tan 0 E 0, the above equation reduces to the formwhich was assumed in copending application, Ser. No. 858,002, (WatkinsCase 1), namely,

x nAf/d FIG. 7 is a graph showing the distance from the origin (center)of the opaque filter regions, as a function of the order of the spatialfrequency, for the linear equation assumed in the copending aplication,and for the actual equation given in Equation 2 above. It will beobserved that for the first few orders, the deviation between the lineargraph and the actual, approximately tangential graph is very small, buttowards the higher orders, this discrepancy becomes increasingly larger.

The upper half of FIG. 8 depicts the uniform regionto-region spacingemployed in prior art spatial filters, corresponding to the linear graph47 in FIG. 7. Copending application R. A. Heinz, et al., Ser. No.249,983, of even date, discloses the use of a spatial filter in whichthe region-to-region spacing is not uniform but, rather, increasesaccording to the tangential-like curve 48 in FIG. 7. Thus, as shown inthe lower half of FIG. 8, while the first few opaque regions in thefilter disclosed in the copending application, R. A. Heinz, et al., CaseI- 1 -4-1 of even date, are at approximately the same position as theywould be for the linear case, if one moves outward, to the right, fromthe center of the filter, the discrepancy between the position of theregions in the linear filter and those in the non-linear filter of thecopending application, R. A. Heinz, et al., Case l-l-4-l of even date,becomes increasingly large. Again, it must be emphasized that forclarity, the scale has been greatly exaggerated.

Because the step-and-repeat spacing of typical integrated circuitdevices varies from to 120 mils, the typical spacing between the opaqueregions on a spatial filter varies from 20 to l20 microns, assuming anHeNe laser and a 100 mm focal length lens. It is, therefore,

essential that the filter be manufactured with the greatest care, andconsiderable accuracy is required to successively increase the distancebetween the regions, in accordance with Equation 2. Accordingly, if thespatial filter disclosed in copending application, R. A. Heinz, et al.,Case 1-l-4-l of even date, constructed in accordance with Equation 2 andgraph 48 of FIG. 7, is substituted for the spatial filter 23 in FIG. 1,the filter will effectively block all repetitive information from thephotomask 17, including the edge information, even though the regionsare actually positioned on planar surface 45, rather than the actualback plane of lens 22.

From a practical standpoint, the requirements for manufacturing thefilter described in copending application, R. A. Heinz, et al., Case1-1-4-1, of even date, are so demanding, particularly the progressive,but minute, increase in region-toregion spacing, that production canonly be satisfactorily accomplished in a computer-controlled device suchas the Primary Pattern Generator or in a computer-controlledelectronbeam machine. This, of course, makes the filter relativelyexpensive and time consuming to produce. Fortunately, for less criticalapplications, where a certain degree of extraneous high-frequency edgeinformation can be tolerated, the filter of the instant invention can besubstituted for the filter used in said copending application, R. A.Heinz, et al., Case l-l-4-l, of even date.

As shown in FIG. 9, we have discovered if a line 49 is drawn having aslope slightly steeper than that of line 47, which latter linecorresponds to the linear Equation 3 above, the line will intersect thetangential-like curve at the origin and at the point 51. Thus, accordingto the invention, if a spatial filter is constructed having a uniformregion-to-region spacing defined by line 49, rather than line 47 or 48,the opaque filter regions lying to the left of point 51 will be locatedfurther from the origin than the theoretical location would dictate, theregions which are clustered about point 51 will be positioned extremelyclose to the theoretically defined positions, and the regions to theright of 51 will be closer together and closer to the origin than thetheoretical location would dictate.

Now, if the opaque regions on the filter are somewhat larger than thespots of light in the diffraction pattern of the workpiece to beexamined, the filter regions to the left of point 51 will, nevertheless,intercept the periodic information in the diffraction pattern.

Assume that the point 52 on graph 47 corresponds to the position of thatopaque region on filter 23, which no longer satisfactorily interceptsdiffracted light from the mask 17. The vertical distance from' thispoint to the corresponding point 53' on graph 48 represents thedeviation x between the actual position of this opaque region and theposition that this same order region would have occupied had theregionto-region spacing on the filter been steadily increased, accordingto graph 48 and Equation 2. In a similar manner, point 54 on graph 49represents the location of the last opaque region to satisfactorilyblock diffracted light from pattern 17 in a filter constructed accordingto this invention. Again x, represents the deviation between the trueposition of this region and the position that it would have occupied hadthe region-to-region spacing been steadily increased according toEquation 2. It will be observed that for the same deviation, i.e., whenx x,, the location 54 of the last satisfactory opaque region on a filteraccording to this invention, lies much further to the right than thelocation 53 on a filter constructed according to the prior artteachings. In other words, even though the region-to-region spacing onthe instant filter is uniform, and deviates from the optimum ortheoretical spacing, the results obtained by the use of this filterwill, nevertheless, be significantly superior to those obtaincd by theuse of the filters disclosed in the abovediscussed copending application(Watkins Case 1). The reason for this is that a larger percentage of theopaque filter regions are positioned to effectively block the periodicinformation, and hence, suppress highfrequency edge information. This isillustrated in more detail in FIG. in which a plurality of opaque filterregions 36 are drawn in alignment with the graph of FIG. 9. For clarity,opaque regions 36 have been assumed to be square. The black regionswithin each such filter regions are intended to represent the brightspots of light generated by the Fourier transform of photomask 17. Ascan be seen from the figure, the center opaque filter region ispositioned to intercept the dot of light corresponding to the dc. termsquarely in the center thereof. This is no surprise because thedeviation between graph 48 and graph 49 is zero for the dc term.However, as the order increases, in a direction to the right in FIG. 10,the spots of light are intercepted by the corresponding filter regionsmore and more to the left of center until at the fourth filter regionshown, the lightspot almost fails to be intercepted. Continuing on,however, for higher and higher orders, the light spot begins to move tothe right until, at the filter region corresponding to point 51 on thegraph (i.e., at the intersection of graphs 48 and 49), the light dot isonce more squarely centered in the filter region. Continuing on to stillhigher orders, the point of interception moves to the right until, asshown, in the last filter region, the light dot completely fails to beintercepted and from then on, the filter will fail to suppresshigh-frequency edge information, until thelight dot again gets insynchronism with the opaque filter regions.

The apparent movement of the light dot, with respect to the filterregions is shown in FIG. 11. As will be selfevident, the dot appears tomove from the center of the opaque region to the extreme left and, then,reverses direction and moves to the right passing once more through theexact center of the opaque filter region. It will be appreciated thatFIGS. 9 and 10 are not to scale and are merely illustrative of theoperation of the invention. The motion of the light spots shown in FIGS.9 and 10 actually occurs over several hundred opaque filter regions,rather than the dozen or so shown.

Theoretically, by the. use of the filter according to this invention,nearly twice as many light spots can be successfully intercepted thencan be intercepted by the linear prior art filter disclosed in copendingapplication, Ser. No. 858,002, (Watkins Case 1), that is to say,

in a typical filter, approximately 200 or more light reglOnS.

One skilled in the art will appreciate that while the invention has beendescribed with reference tothe inspection of integrated circuitphotomasks, it may also be used to inspect any workpiece having opticalcharacteristics approximating those of an optical grating, eithertransmissive or reflected, e.g., a processed silicon semiconductorslice. For example, the invention has successfully been used to inspectfine metallic grids, and diode array targets, such as those used in themanufacture of Picturephone camera tubes, and the like. Further, ifdesired, the spatial filter might comprise a matrix oftransparentregions on an opaque field rather than a matrix of opaqueregions on a transparent field. In this latter event, periodicinformation would be transmitted, rather than blocked. Of course, theterm regions, as used herein, is intended to comprise various shapes,such as circles, squares, triangles, etc. The

actual shape employed is merely a matter of convenience, provided thatthe corresponding light dot in the diffraction pattern is blocked (orpassed). Also, various changes and substitutions may be made to theelements shown, without departing from the spirit and scope of theinvention.

Finally, it must again be stressed, that while Manhattan geometry is byfar the most common found in integrated circuits, the methods andapparatus of this invention may be used to inspect workpieces having anygeometry in their features.

What is claimed is:

l. A method of isolating non-periodic errors in a twodimensional patterncontaining a regular array of nominally identical regions, mutuallyspaced apart by a predetermined distance along at least one axis, whichcomprises the steps ofi directing a spatially coherent beam of light atthe pattern to diffract the light;

focusing the diffracted light on a filter consisting of a plurality ofdiscrete substantially equally sized opaque regions on a transparentfield, the spacing between adjacentregions, along at least one axis ofthe filter being uniform and greater than the spacing dictated by theequation:

where,

x the distance of the n region from the origin, A the wavelength of saidbeam of light, n the order of the spatial harmonic, d thestep-and-repeat distance of said regular array of regions,

f the focal length of said focusing lens,

wherein at least one of said regions is positioned in coincidence. witha location dictated by the equa tion:

where,

x the distance of the n" region from the centermost region (origin); )tthe wavelength of said beam of light; n the order of the spatialharmonic. and is greater than one; d. the step-and-repeat distance ofsaid regular array of regions, 5 f the focal length of said focusinglens; to spatially modulate the light; and

reimaging the spatially modulated light to form an image exhibiting thenon-periodic errors in the pattern, said filter blocking essentially allperiodic information in said image, including higher spatial frequencycomponents.

2. The method according to claim 1 wherein said point of coincidencelies approximately halfway between the centermost filter regions and theedge of said filter.

3. Apparatus for inspecting non-periodic errors in a two-dimensionalpattern containing a plurality of nominally identical and regular spacedregions arranged in a planar periodic array, which comprises:

means for directing a spatially coherent beam of light at the plane ofthe pattern so that the light is diffracted thereby;

a first lens positioned to focus the light diffracted by the pattern;

a planar optical filter consisting of a distribution of discretesubstantially equally sized opaque regions on a transparent field, thespacing between adjacent regions, along at least one axis of the filterbeing uniform and greater than the spacing dictated by the formula:

where,

x the distance of the n" region from the centermost region (origin),

A the wavelength of said beam of light,

n the order of the spatial harmonic,

d the step-and-repeat distance of said regular array of regions,

f the focal length of said focusing lens,

wherein at least one of said regions is positioned in coincidence with alocation dictated by the equation:

1: =ftan [sin (nM/d] where,

x the distance of the n" region from the centermost region (origin); Athe wavelength of said beam of light; n the order of the spatialharmonic and is greater than one; d the step-and-repeat distance of saidregular array of regions; f the focal length of said focusing lens; thefilter being positioned at the focal plane of the first lens forspatially modulating the intensity of the light focused thereon by thefirst lens;

a second lens positioned to reimage the light transmitted by the filterto form a visual image of the non-periodic errors in the pattern; and

means for projecting the visual image onto the image display means.

4. Apparatus according to claim 3 wherein said image display means andsaid projecting means comprises:

a television camera focused on said visual image;

control means, connected to said camera, for supplying deflectionsignals and power to said camera, said camera scanning across saidvisual image to detect said non-periodic errors; and

counting means, connected to the video output of said camera, forcounting the number of nonperiodic errors so detected.

5. A spatial filter for filtering the Fourier transform of the image ofa workpiece comprising a matrix-like array of nominally identicalfeatures, which consists of:

a matrix-like array of discrete substantially equally sized opaqueregions on a transparent field, the

spacing between adjacent regions, along at least one axis of said array,being uniform and greater than the spacing dictated by the equation:

where,

x the distance of the n"' region from the centermost region (origin);

A the wavelength of the light forming said image; n the order of thespatial harmonic; d the step-and-repeat distance of the workpiece; f thefocal length of the Fourier transform lens;

wherein at least one of said regions is positioned in coicidence with alocation dictated by the equation;

where,

x the distance of the 11" region from the centermost region (origin);

A the wavelength of the light forming said image;

n the order of the spatial harmonic and is greater than one;

d the step-and-repeat distance of the workpiece;

f the focal length of the Fourier transform lens; whereby said opaqueregions inhibit further transmission of substantially all periodicinformation in said transform.

6. The filter according to claim 5 wherein said point of coincidencelies approximately halfway between the centermost filter region and theedge of said filter.

. i i i i i

1. A method of isolating non-periodic errors in a twodimensional patterncontaining a regular array of nominally identical regions, mutuallyspaced apart by a predetermined distance along at least one axis, whichcomprises the steps of: directing a spatially coherent beam of light atthe pattern to diffract the light; focusing the diffracted light on afilter consisting of a plurality of discrete substantially equally sizedopaque regions on a transparent field, the spacing between adjacentregions, along at least one axis of the filter being uniform and greaterthan the spacing dictated by the equation: x n lambda f/d where, x thedistance of the nth region from the origin, lambda the wavelength ofsaid beam of light, n the order of the spatial harmonic, d thestep-and-repeat distance of said regular array of regions, f the focallength of said focusing lens, wherein at least one of said regions ispositioned in coincidence with a location dictated by the equation: x ftan (sin 1 (n lambda )/d) where, x the distance of the nth region fromthe centermost region (origin); lambda the wavelength of said beam oflight; n the order of the spatial harmonic and is greater than one; dthe step-and-repeat distance of said regular array of regions; f thefocal length of said focusing lens; to spatially modulate the light; andreimaging the spatially modulated light to form an image exhibiting thenon-periodic errors in the pattern, said filter blocking essentially allperiodic information in said image, including higher spatial frequencycomponents.
 2. The method according to claim 1 wherein said point ofcoincidence lies approximately halfway between the centerMost filterregions and the edge of said filter.
 3. Apparatus for inspectingnon-periodic errors in a two-dimensional pattern containing a pluralityof nominally identical and regular spaced regions arranged in a planarperiodic array, which comprises: means for directing a spatiallycoherent beam of light at the plane of the pattern so that the light isdiffracted thereby; a first lens positioned to focus the lightdiffracted by the pattern; a planar optical filter consisting of adistribution of discrete substantially equally sized opaque regions on atransparent field, the spacing between adjacent regions, along at leastone axis of the filter being uniform and greater than the spacingdictated by the formula: x n lambda f/d where, x the distance of the nthregion from the centermost region (origin), lambda the wavelength ofsaid beam of light, n the order of the spatial harmonic, d thestep-and-repeat distance of said regular array of regions, f the focallength of said focusing lens, wherein at least one of said regions ispositioned in coincidence with a location dictated by the equation: x ftan (sin 1 (n lambda )/d) where, x the distance of the nth region fromthe centermost region (origin); lambda the wavelength of said beam oflight; n the order of the spatial harmonic and is greater than one; dthe step-and-repeat distance of said regular array of regions; f thefocal length of said focusing lens; the filter being positioned at thefocal plane of the first lens for spatially modulating the intensity ofthe light focused thereon by the first lens; a second lens positioned toreimage the light transmitted by the filter to form a visual image ofthe non-periodic errors in the pattern; and means for projecting thevisual image onto the image display means.
 4. Apparatus according toclaim 3 wherein said image display means and said projecting meanscomprises: a television camera focused on said visual image; controlmeans, connected to said camera, for supplying deflection signals andpower to said camera, said camera scanning across said visual image todetect said non-periodic errors; and counting means, connected to thevideo output of said camera, for counting the number of non-periodicerrors so detected.
 5. A spatial filter for filtering the Fouriertransform of the image of a workpiece comprising a matrix-like array ofnominally identical features, which consists of: a matrix-like array ofdiscrete substantially equally sized opaque regions on a transparentfield, the spacing between adjacent regions, along at least one axis ofsaid array, being uniform and greater than the spacing dictated by theequation: x n lambda f/d where, x the distance of the nth region fromthe centermost region (origin); lambda the wavelength of the lightforming said image; n the order of the spatial harmonic; d thestep-and-repeat distance of the workpiece; f the focal length of theFourier transform lens; wherein at least one of said regions ispositioned in coicidence with a location dictated by the equation; x ftan (sin 1 (n lambda )/d) where, x the distance of the nth region fromthe centermost region (origin); lambda the wavelength of the lightforming said image; n the order of the spatial harmonic and is greaterthan one; d the step-and-repeat distance of the workpiece; f the focallength of the Fourier transform lens; whereby said opaque regionsinhibit further transmission of substantially all periodic informationIn said transform.
 6. The filter according to claim 5 wherein said pointof coincidence lies approximately halfway between the centermost filterregion and the edge of said filter.